This invention relates to a combined cycle power plant realized by consolidating a gas turbine system and a steam cycle system, each being provided with one or more than one turbine.
More specifically, the present invention relates to a combined cycle power plant designed to improve the overall thermal efficiency of the plant by collecting the waste heat of the gas turbine system in said steam cycle system and enhancing the efficiency of collecting the waste heat of said gas turbine system.
There are known combined cycle power plants comprising a gas turbine system and a steam cycle system, wherein the steam cycle system is bled of steam and the bled steam is used as a cooling medium for cooling the turbine blades of the gas turbine while the heat collected by the cooling operation is returned to the steam cycle system to improve the overall thermal efficiency of the plant.
Combined power plants of the above identified type are advantageous in that the turbine inlet gas temperature can be raised to improve the thermal efficiency of the turbine as a result of the cooling operation using steam as a cooling medium and, at the same time, the waste heat of the gas turbine system is partly collected and returned to the steam cycle system to improve the overall thermal efficiency of the plant.
FIG. 1 of the accompanying drawings schematically illustrates a known combined cycle power plant comprising a gas turbine system 1 and a steam cycle system 2 having a steam turbine driven by the waste heat of the gas turbine system 1.
The gas turbine system 1 by turn comprises a gas turbine 11, a compressor 13 coupled with the gas turbine 11 by means of a shaft 12 and a combustor 14 that receives compressed air and fuel from the compressor 13 and burns the fuel to produce hot high pressure gas for driving the gas turbine 11.
The compressor 13 compresses the ambient air fed in through an air duct 15 and the compressed air is partly used for cooling the blades and sealing the rotary components of the gas turbine 11, while the remaining compressed air is fed to the combustor 14.
The combustor 14 burns the fuel fed from a fuel supply system (not shown), using the compressed air as combustion sustaining gas. The hot gas obtained by combustion is then fed to the gas turbine 11 by way of a combustion gas duct 16 and allowed to inflate to drive the gas turbine 11 before flowing out through an exhaust gas duct 17.
On the other hand, the steam cycle system 2 comprises a steam turbine 18 and an electric generator 20 coupled with the steam turbine 18 by means of a shaft 19. A steam cycle is established in the steam cycle system as it generates steam by the waste heat of the gas turbine system 1 and the generated steam is used to drive the steam turbine 18. Note that the rotor of the steam turbine 18 and that of the gas turbine 11 are linked by a shaft 22 in FIG. 1.
The steam cycle system 2 is additionally provided with a waste heat collector boiler 23 for collecting heat from the exhaust of the gas turbine 11 fed through the exhaust gas duct 17 and generating hot high pressure steam necessary for driving the steam turbine 18. The exhaust gas from the waste heat collector boiler 23 is then emitted into the atmosphere by way of a flue 27.
A high pressure superheater 24, a high pressure evaporator 25 and a high pressure preheater 26 are arranged in the waste heat collector boiler 23 in the above mentioned order from the upstream side toward the downstream side thereof. These heaters and the steam turbine 18 are connected with each other in a manner as described below to establish a steam cycle 2.
The steam expelled from the steam turbine 18 is led to a steam condenser 29 by way of an expelled steam duct 28 and reduced to water at room temperature. The collected room temperature water is led to the high pressure preheater 26 by way of a circulation pump 30 and a circulation conduit 31 for preliminary heating and then to a high pressure drum 33 by way of a conduit 32 leading to the drum.
The pressurized water in the high pressure drum 33 is led to the high pressure evaporator 25 by way of a pressurized water circulation pump 34 and a pressurized water circulation conduit 35 for evaporation and the produced hot high pressure steam is returned to an upper space of the high pressure drum 33 by way of a return duct 36. The returned steam is then fed to the high pressure superheater 24 by way of a steam duct 37 for reheating and the heated steam is further fed to the steam turbine 18 by way of a steam supply duct 38.
In a combined cycle power plant having a configuration as described above, the inlet gas temperature of the gas turbine 11 is preferably made as high as possible in order to enhance the thermal efficiency of the plant. On the other hand, the combustor 14 and the stator vanes and the rotor blades of the gas turbine 11 have to be made of a highly heat-resistant material to achieve a high inlet gas temperature for the gas turbine 11.
The upper temperature limit of currently available heat-resistant super alloys that can be used for gas turbines is about 800 to 900.degree. C. On the other hand, the inlet gas temperature of newly constructed gas turbines is typically as high as about 1,300.degree. C., a level far exceeding the upper temperature limit of heat-resistance super alloys. Therefore, the temperature of the blades of the turbine 11 has to be lowered by some means to the upper temperature limit of the heat-resistant super alloy of which they are made. In the case of gas turbines having an inlet gas temperature of about 1,300.degree. C., the blades are typically cooled by part of the air expelled from the compressor 13.
However, Air-cooling systems using air as cooling medium intrinsically show a rather poor effect. Therefore, cooling air has to be made to flow at a remarkably high rate to cool the blades of the turbine if the inlet gas temperature rises above 1,300.degree. C. Further more, the convection cooling inside the blades alone cannot achieve a sufficient cooling effect and a film cooling technique of blowing out cooling air through pores arranged on the surface of the effective area of each blade has to be used as supplementary means.
The use of a film cooling technique is accompanied by a problem of reduced temperature of main stream gas as the blown out cooling air and the main stream gas are mixed with each other. This means that the combustor 14 has to be designed so as to withstand a higher outlet temperature and, at the same time, suppress the emission of NOx gas at such high temperature. If these design requirements are met, the combustor 14 consumes air and fuel at an enhanced rate.
As discussed above, the gas turbine having an air cooling system of a combined cycle power plant is inevitably accompanied by the problem of reduced thermal efficiency, which by turn reduces the overall thermal efficiency of the plant. Additionally, the pores on the surface of the blades can become clogged when low quality fuel containing impurities is used so that the plant has to be sensitive about the fuel it consumes in order to avoid the use of low quality fuel.
Japanese Patent Publication No. 63-40244 and Japanese Patent Application Laid-Open No. 4-124414 disclose techniques of using steam whose specific heat is twice as large as that of air as cooling medium in a gas turbine in order to bypass the above identified problems. With any of the proposed techniques, part of the steam to be used for the steam turbine of a combined cycle power plant is made to flow through the cooling duct arranged in the blades of the gas turbine to cool the blades and, after cooling the blades, put back to the remaining steam that is being fed to the steam turbine.
A combined cycle power plant realized by applying such a technique consumes steam at a rate far below the air consumption rate of a comparable plant that does not use the technique and achieves a desirable cooling effect without blowing out steam through the blades of the gas turbine. The steam used for cooling the blades is collected and roused to drive the steam turbine. Thus, the use of such a technique can effectively prevent any significant temperature fall in the main stream gas and a rise in the consumption rate of fuel and air within the combustor to improve the overall thermal efficiency of the plant. Additionally, the gas turbine can be adapted to low quality fuel.
However, in any known combined cycle power plants designed to cool the blades of the gas turbine with steam by allowing part of the steam for the steam turbine to flow through the cooling duct arranged in the gas turbine blades and thereafter to be combined with the remaining steam, the two parts of steam are actually put together at the inlet of the steam turbine before fed to it so that it is rather difficult to accurately control the steam flow rate, the steam pressure and the steam temperature to get to respective target values at the inlet of the steam turbine and achieve a maximum thermal efficiency for the plant.
Japanese Patent Application Laid-Open No. 6-93810 proposes a technique of dissolving this problem by returning substantially the entire steam that has passed through the cooling duct to the heating region of the steam cycle system.
The combined cycle power plant realized by applying the above technique, however, has a drawback at the time of starting and stopping the operation and during a partial load operation where steam for cooling the gas turbine system is often difficult to supply. For instance, when the plant is started cold, the steam does not come out from the waste heat collector boiler immediately after the gas turbine is activated. Thus, a cooling process of the turbine blades cannot be conducted until the steam is ready for supplying. In order to make up for this steam deficiency at the beginning and end of the operation, this type plant requires an auxiliary boiler, which results in a high manufacturing cost.
Furthermore, in the plant of the type taught in Japanese Patent Application Laid-Open No. 6-93810, the steam passes through the cooling ducts in the blades of the gas turbine and other components so that water is trapped in the ducts after the plant is stopped. This water needs to be drained out by using steam or high pressure gas when the plant is restarted. Such an operation requires a high-pressure gas source, which also increases the cost for building the plant.